The present invention relates to a filtration method for filtering solid suspended within sewage water.
In order to remove solids suspended in sewage water, various methods and apparatus with the separation of liquid and solids have been employed according to the particular properties of the solids involved.
For the treatment of sewage water containing a substantial amount of suspended solids, various precipitation tanks have been employed. In order to concentrate solids separated by a precipitation tank, various dehydrators, for example a centrifugal dehydrator and a filter press machine, have been used according to the particular properties of the solids involved.
The precipitation tanks, in general, have been of the type employing a large plane area for their location. Since the separation capacity of a precipitation tank depends upon the sedimentation speed of the solids therein, it has been found necessary to employ a large sedimentation area, that is, a large plane area for the location to increase the separation capacity. When the sedimentation area of the precipitation tank is small, the quality of the treated water becomes unsatisfactory.
Therefore, it has been a requirement that precipitation tanks have a large plane area for their location. Filtration methods and apparatus to take the place of such filtration tanks and avoid the problems of large areas have been attempted. However, prior attempts have employed filters that have had the defect that the filters become blocked quickly by the attachment of solid to the filter material surface.
For example, there is known a method and apparatus such that the period of time when the filter is blocked is obtained theoretically and through experience, so that after such time has passed, the filtration operation is interruped and the filter is washed to remove the attached solids. Such stoppage of filtration and washing will prevent the clogging or blocking of the filter, but has the disadvantages of waste down time.
Furthermore, there has been provided another method employing two filtration tanks having filter materials. According to this method and apparatus, while one of the filtration tanks is used for filtration, the filter of the other filtration tank is being washed; thereafter, the filtration tank that has just been washed is used for filtration while the remaining filtration tank is washed. This operation employing two filtration tanks has the advantage that filtration is continuous, whereas the washing of only one filtration tank requires an interruption in the filtering.
However, both of the operations employing the filtration tanks require a period for the washing operation of a filter that has been used, during which time the filter may not be used for filtration. As mentioned above, blocking of the filter prevents the continuous filtration of suspended solids contained in sewage water, because in order to avoid such blocking it is necessary to remove the solids attached to the surface of the filter before the solids block the filter, which removal has been accomplished only without further filtration.